The present invention relates to the class of DC-to-DC converters that incorporate the topology represented in FIG. 1. A converter in that class is referred to as a "single ended forward" converter because power flow is gated by a single switch and energy is transferred forward, from the primary winding to the secondary winding of the transformer, during the ON period of the switch. Converters in this class present a unique problem, in that the conversion topology does not inherently define the mechanism by which the transformer's core is to be reset during the OFF period of the switch. A number of solutions are proposed in the prior art, each with different tradeoffs in the cost of the converter, as well as its efficiency and power density.
A well-known prior art DC/DC forward converter is shown in FIG. 2A, where L.sub.k, D.sub.s and C.sub.ds are the leakage inductance of the transformer T, body diode and internal capacitance of switch S, respectively. Winding N3, coupled with windings N1 and N2, is used to reset the transformer T. The typical switching waveforms of FIG. 2A are shown in FIG. 2B. When switch S is turned off at t.sub.2, energy stored in the leakage inductance L.sub.k is released to charge the capacitance C.sub.ds, which causes a high voltage spike across switch S. After the leakage energy is completely released, the voltage across switch S reaches its steady-state value. As a result, high voltage rating voltage S would be required.
To eliminate this voltage spike, a number of circuit topologies have been reported in the literature. Among them, the R-C-D snubber is one of the most popular ways to minimize the voltage spike as shown in FIG. 3. The snubber circuit consists of diode D1, capacitor C.sub.s and resistor R.sub.s. When switch S is turned off, the leakage current flows through diode D1 and charges capacitance C.sub.s. If capacitance C.sub.s is relatively large enough, the voltage across C.sub.s roughly does not change so as to clamp the voltage. In this case, the leakage energy of the transformer is first charged to C.sub.s and then is dissipated by the resistor R.sub.s. As a result the converter has lower conversion efficiency, i.e., loss of the energy inherent in the spike to heat.
The leakage inductance of the transformer in a conventional DC/DC forward converter causes a voltage spike across the power switch when the power switch turns off. Usually a circuit, such as R-C-D (resistor, capacitor, diode) snubber or an active clamp circuit, is used to absorb this voltage spike. The leakage energy of the transformer is dissipated in the R-C-D snubber circuit.
A number of known designs seek to recover this energy. These methods typically require an additional active switch to recover the leakage energy of the transformer.
See, Moshe Domb, "Nondissipative turn-off snubber alleviates switching power dissipation second-breakdown stress and Vce overshoot: analysis, design procedure and experimental verification," IEEE Power Electronics Specialists Conference (1982); U.S. Pat. No. 4,783,727, "DC/DC Converter"; U.S. Pat. No. 6,115,271, "Switching Power Converters With Improved Lossless Snubber Networks", U.S. Pat. No. 5,260,607, "Snubber Circuit For Power Converter", each of which is incorporated herein by reference.
Farrington, U.S. Pat. No. 5,883,795; Farrington, U.S. Pat. No. 5,883,793; and Gautherin et al., U.S. Pat. No. 4,675,796, each of which is expressly incorporated herein by reference, are discussed below.
See also, R. Watson, F. C. Lee and G. C. Hua, "Utilization of an active clamp circuit to achieve soft-switching in flyback converters" IEEE Power Electronics Specialists Conference (1994).
U.S. Pat. No. 6,108,218, "Switching Power Supply with Power Factor Control", provides two embodiments. In a first embodiment, shown in FIGS. 1 and 2 thereof, no snubber circuit to recycle the leakage energy of the transformer is shown. FIGS. 3 and 4 thereof provide an additional active switch as part of the snubber.
U.S. Pat. No. 6,061,254, "Forward Converter With Active Clamp Circuit", provides a circuit having three inductively linked transformer windings and at least two active switches.
U.S. Pat. No. 5,982,638, "Single stage power converter with regenerative snubber and power factor correction" provides a capacitor 44 in FIG. 1, which is not only used as a snubber capacitor, but also used to achieve power factor correction, and therefore handles the main power flow from the input to the output. Therefore, the current flowing through this capacitor 44 is very large, which requires a capacitor large in size and value. In this circuit, the recovery of energy from the snubber capacitor 44 occurs by transfer to the input inductor 38 when switch 22 turns on. Since the energy stored in capacitor 44 is large, which causes higher power loss in the circuit. As a result, it has lower power conversion efficiency. The capacitance of capacitor 44 is determined by the input power and satisfies the power factor and input current harmonics requirements.
U.S. Pat. No. 5,991,172, "AC/DC flyback converter with improved power factor and reduced switching loss," provides a third transformer winding which is not used to recover the leakage energy of the transformer, but rather to reduce the switching loss and improve the power factor. The leakage energy is dissipated by the circuit. Thus, it provides no substantial improvement in efficiency over a dissipative R-C-D snubber.
U.S. Pat. No. 5,999,419, "Non-isolated Boost Converter With Current Steering" relates to a buck boost converter having a tree-winding transformer.
U.S. Pat. No. 5,896,284, "Switching Power Supply Apparatus With a Return Circuit That Provides A Return Energy Ro A Load", relates to a power supply circuit which utilizes leakage inductance energy to enhance efficiency, for example with a magnetically isolated inductor.
U.S. Pat. No. 5,615,094, "Non-Dissipative Snubber Circuit For A Switched Mode Power Supply", relates to a snubber circuit for a secondary circuit of a power supply.
U.S. Pat. No. 5,694,304, "High Efficiency Resonant Switching Converters"; and U.S. Pat. No. 5,379,206, "Low Loss Snubber Circuit With Active recovery Switch" each provide a dual active switch architecture converter.
U.S. Pat. No. 5,055,991, "Lossless Snubber", relates to a converter circuit having an active switch and a transformer with five inductively coupled windings.
U.S. Pat. No. 5,019,957, "Forward Converter Type of Switched Power Supply", relates to a dual active switch forward power converter.
U.S. Pat. No. 4,805,079, "Switched Voltage Converter", provides a converter with a snubber circuit.
U.S. Pat. No. 4,760,512, "Circuit for Reducing Transistor Stress and Resetting the Transformer Core of a Power Converter", relates to a single active switch, triple inductively coupled winding transformer forward converter.
U.S. Pat. No. 4,736,285 relates to a "Demagnetization circuit for Forward Converter", having two active switches.
U.S. Pat. No. 4,688,160, "Single Ended Forward Converter With Resonant Commutation of Magnetizing Current", provides a forward converter employing a resonating capacitor to reset the transformer core.
U.S. Pat. No. 4,561,046, "Single Transistor Forward Converter With Lossless magnetic Core Reset and Snubber Network", relates to a forward converter having a single switch and a transformer having three inductively linked windings.
U.S. Pat. No. 4,441,146, "optimal Resetting of the Transformer's Core in Single Ended Forward Converters", provides a forward DC/DC converter having a transformer with three inductively coupled windings.
U.S. Pat. No. 4,355,352, "DC To DC Converter", relates to a converter having three coupled inductor windings, with two capacitors and two switching devices (one active and one passive), to provides a ripple free input and output current.